Separation of blood into a plasma fraction and a cellular component fraction is desirable for many medical reasons. For example, separation of blood into plasma fractions and cellular component fractions provides for a collection of plasma alone, with the cellular component being returned to the donor with an optional suitable portion of replacement fluid. Thus continuous plasmapheresis provides for the collection of plasma from donors without the removal of the cellular components of the blood. Plasma donation from a patient or donor is generally allowed about twice a week whereas the whole blood donation is allowed once in every two months. Secondly, continuous plasmapheresis can be used therapeutically to remove pathologic substances contained in the plasma portion of the blood, as disclosed by Popovich et al. in U.S. Pat. No. 4,191,182. This can be accomplished by separating the cellular components from the diseased plasma and returning the cellular components to the patient in admixture with a suitable replacement fluid, or by further fractionating the patient's plasma to remove the unwanted substances and returning a major portion of the patient's plasma with the cellular components.
The separation of blood into cellular component fractions and plasma fractions has inherently some difficulties and complications. A brief discussion of the makeup of blood is shown herein for illustration purposes. Approximately 45% of the volume of blood is in the form of cellular components. These cellular components include red cells, white cells and platelets. If cellular components are not handled correctly, the cells may lose their functionality and become useless. Plasma makes up the remaining 55% of the volume of blood. Basically, plasma is the fluid portion of the blood which suspends the cells and comprises a solution of approximately 90% water, 7% protein and 3% of various other organic and inorganic solutes. As used herein, the term “plasmapheresis” refers to the separation of a portion of the plasma fraction of the blood from the cellular components thereof.
Ultrafiltration has been widely used on a batch-type or continuous basis as a substitute for, or in combination with, dialysis methods in artificial kidneys and the like. In any plasmapheresis-type process effected by ultrafiltration there are various problems which occur during the fractionating of the blood by passing it in a parallel flow pattern over a membrane surface, with a transmembrane pressure sufficient to push the plasma portion of the blood therethrough, while allowing the cellular component portion of the blood to remain thereon. One of these problems is that the flow rates must be controlled fairly closely. Thus, if the flow rate employed is too fast at any moment or at any specific region, detrimental turbulence may occur and excess shear force may cause unwanted hemolysis resulting in general destruction of cellular components. On the other hand, if the flow rate and the transmembrane pressure are not controlled adequately the cellular and macromolecular components of the blood will tend to clog up the membrane thus significantly slowing the ultrafiltration rate. Such clogging can also cause hemolysis to occur.
Along the blood flow route in a plasmapheresis apparatus, plasma continues to pass through the filter membrane while cellular component remains in the blood stream. At the downstream region of the separation process, the blood becomes more viscous and the separation efficiency decreases drastically. This fouling effect or “concentration polarization” phenomenon becomes obvious in a conventional batch-wise or continuous ultrafiltration process. For example, U.S. Pat. No. 3,705,100 to Blatt et al., issued Dec. 5, 1972, discloses a process and apparatus for a blood fractionating process on a batch basis. Furthermore, U.S. Pat. No. 4,191,182 to Popovich et al., issued Mar. 4, 1980, discloses a means for continuous plasmapheresis including a blood input pumping means and a plasma outflow pumping means. Though the average flow rate of the disclosed device is within the non-hemolysis range, the local flow rate and its shear force at any moment and/or at any specific region of the filter membrane may not be adequate to effect the most efficient plasmapheresis. Concentration polarization usually occurs at a later stage in a batch plasmapheresis or at a downstream region in a continuous plasmapheresis.
To compensate for the concentration polarization drawbacks, Solomon et al. in U.S. Pat. No. 4,212,742 discloses a filtration device employing a microporous filtration membrane. The filtration flow channels along the surface of the upstream side of the membrane wall are provided with gradually and uniformly increases from the inlet end to the outlet end of the flow channel, whereby the membrane wall shear force of the suspension in laminar flow through the flow channel gradually and uniformly varies along the length of the flow channel from a maximum value at its inlet end to a minimum value at its outlet end. However, Solomon et al. device requires enormous membrane surfaces for blood plasma separation which appear not economically practical.
For the purposes of increasing the transmembrane pressure drop hopefully to catch a higher separation efficiency and a less concentration polarization effect, Fischel in U.S. Pat. No. 5,034,135, Schoendorfer in U.S. Pat. No. 5,194,145, Duff in U.S. Pat. No. 5,234,608, Fischel in U.S. Pat. No. 5,376,263, and Brown in U.S. Pat. No. 5,529,691 all disclose a blood separating system comprising high rotational velocity flow applying centrifugal forces aiming for added transmembrane pressure drop. During high centrifugal rotation, a portion of the cellular components may undesirably remain in the rotational device or inside pores of the filter membrane for a prolonged time and may subject to hemolysis, cellular damage or membrane clogging. For centrifugal-type separation processes, the local shear force for the cellular components of the blood concentrate fraction is the highest at about the periphery of the separation apparatus, such as a spinner-type device and the like. The requirement of a proper shear force at the outer-most region in a rotational separator apparently limits the size, and therefore the capacity, of the separation apparatus or the spinner.
Alternately, to create adequate local flow rate and subsequently local shear force in a plasmapheresis process, Duggins in U.S. Pat. No. 4,735,726 discloses a process for continuous plasmapheresis comprising conducting blood over a microporous membrane in a reciprocatory pulsatile flow pattern. The pulsatile flow is known to cause certain degrees of turbulence as the pulsatile flow rate changes constantly which may possibly cause cell damage and membrane clogging. Duggins discloses a damage-controlling method to compensate for the shortcomings of the pulsatile flow in a continuous plasmapheresis by reducing the transmembrane pressure difference to below zero during each forward and reverse flow. This additional equipment setup and control mechanism for repetitively reversing the transmembrane pressure difference makes this process less economically attractable.
There is an urgent clinical need to provide an efficient plasmapheresis process by minimizing the cellular damage while increasing the flow output. This may be achievable by controlling the local flow rate and local shear force of a filtration apparatus comprising a filter membrane with an orbital motion to minimize problems of undesired turbulence and concentration polarization in a conventional separating apparatus.